Research Group "Nanochemistry"

A hydrazone-based covalent organic framework for photocatalytic hydrogen production

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Covalent organic frameworks (COFs) are a new generation of porous crystalline polymers. Drawing on the recent development of tailor-made semiconducting COFs, we here report on a new COF capable of visible-light driven hydrogen generation in the presence of Pt as proton reduction catalyst. The COF is based on hydrazone-linked functionalized triazine and phenyl building blocks and adopts a 2D layered structure with a honeycomb-type lattice featuring mesopores of 3.8 nm and the highest surface area among all hydrazone-based COFs reported to date.

Introduction

The last decade has seen a continuous rise in activity revolving around the development of potent photocatalytic systems, which are capable of transforming solar energy into chemical fuels. Whilst most photocatalysts are based on inorganic semiconductors, there are a few examples of materials composed solely of light elements. These systems, prominently represented by carbon nitride polymers, are moderately active in hydrogen generation from water. A closely related class of organic polymers, dubbed covalent organic frameworks (COFs), is apt to overcome these inherent weaknesses of carbon nitrides by combining chemical versatility and modularity with potentially high crystallinity and porosity (see Ref. [1] for more).

Fig. 1: Acetic acid catalysed hydrazone formation furnishes a mesoporous 2D network with a honeycomb-type in plane structure. (a) Scheme showing the condensation of the two monomers to form the TFPT–COF. (b) TFPT–COF with a cofacial orientation of the aromatic building blocks, constituting a close-to eclipsed primitive hexagonal lattice (grey: carbon, blue: nitrogen, red: oxygen). Reproduced from Ref. [3] with permission from The Royal Society of Chemistry.

Fig. 1: Acetic acid catalysed hydrazone formation furnishes a mesoporous 2D network with a honeycomb-type in plane structure. (a) Scheme showing the condensation of the two monomers to form the TFPT–COF. (b) TFPT–COF with a cofacial orientation of the aromatic building blocks, constituting a close-to eclipsed primitive hexagonal lattice (grey: carbon, blue: nitrogen, red: oxygen). Reproduced from Ref. [3] with permission from The Royal Society of Chemistry.

Recently, 2D COFs with interesting optoelectronic properties have emerged, representing ideal scaffolds for exciton separation and charge percolation within self-sorted, nanoscale phase-separated architectures. Whereas most COFs rely on the formation of water-labile boronate ester linkages, a few other examples based on imine and hydrazone linkage have been synthesized recently [2]. Surprisingly, after the pioneering work by Yaghi and coworkers hydrazone formation has not been used again for the synthesis of COFs, although hydrazones are typically much less prone to hydrolysis than imines.

Herein, we report the first COF which is active towards visible light induced hydrogen evolution in the presence of Pt as proton reduction catalyst (PRC). Our hydrazone-based COF (TFPT–COF) is constructed from 1,3,5-tris-(4-formyl-phenyl)triazine (TFPT) and 2,5-diethoxy-terephthalohydrazide (DETH) building blocks (Fig. 1), featuring mesopores of 3.8 nm in diameter and the highest surface area among all hydrazone COFs reported so far.

TFPT–COF: Synthesis and characterization

TFPT-COF was synthesized by the acetic acid catalysed reversible condensation of the building blocks in dioxane/mesitylene (1:2 v/v) at T = 120°C in a sealed pressure vial under argon atmosphere for 72 hours. The product was obtained as a fluffy pale-yellow nanocrystalline solid. To remove any starting material or solvent contained in the pores, TFPT-COF was centrifuged, washed with DMF and THF, soaked in DCM for 3 hours, and subsequently heated to 120°C in high dynamic vacuum for 12 h (10‑7mbar).

Powder X-Ray diffraction (PXRD) measurements confirm the formation of a crystalline framework with metrics being consistent with the structure model shown in Fig. 1. Comparison of the experimental data with the simulation reveal a hexagonal structure with P6/m symmetry and an eclipsed AA layer stacking, which is in line with most COF structures reported to date (Fig. 2(a)).

Fig. 2: Characterization of the TFPT–COF by PXRD and MAS solid-state NMR spectroscopy. (a) and (b) PXRD suggests a (close to) eclipsed layer stacking as confirmed by Pawley refinement of the AA-stacked structure model. (c) Pore size distribution calculated from Argon-sorption data by NLDFT. (d) Argon-sorption isotherms show the formation of mesopores, consistent with the predicted size based on the structure model (see c). The reversible type IV isotherm (adsorption: black triangles, desorption: white triangles) gives a BET surface area of 1603 m2 g–1. (e) TEM images show the formation of a hexagonal pore system; the crystalline domain sizes are on the order of 50–100 nm.Adapted from Ref. [3] with permission from The Royal Society of Chemistry.

Fig. 2: Characterization of the TFPT–COF by PXRD and MAS solid-state NMR spectroscopy. (a) and (b) PXRD suggests a (close to) eclipsed layer stacking as confirmed by Pawley refinement of the AA-stacked structure model. (c) Pore size distribution calculated from Argon-sorption data by NLDFT. (d) Argon-sorption isotherms show the formation of mesopores, consistent with the predicted size based on the structure model (see c). The reversible type IV isotherm (adsorption: black triangles, desorption: white triangles) gives a BET surface area of 1603 m2 g–1. (e) TEM images show the formation of a hexagonal pore system; the crystalline domain sizes are on the order of 50–100 nm.Adapted from Ref. [3] with permission from The Royal Society of Chemistry.

Pawley refinement (including peak broadening) of the experimental powder pattern gave lattice parameters of a = b = 41.90 Å (Fig. 2(a)). The theoretical powder pattern of the related staggered conformation derived from the gra net with P63/m symmetry does not reproduce the observed intensity distribution and was therefore discarded. The 001 diffraction peak at 2θ = 26.6 ° corresponds to an interlayer distance of 3.37 Å (Fig. 2(b)), suggesting a typical van der Waals contact between the aromatic layers. Interestingly, the presence of the ethoxy groups protruding into the pores does not notably increase the interlayer distance, thus indicating a predominantly coplanar arrangement with the plane of the honeycomb lattice.

Argon sorption measurements at T = 87 K clearly show the formation of mesopores as indicated by a typical type IV adsorption isotherm (Fig. 2(d)). The Brunauer-Emmett-Teller (BET) surface area was calculated to be 1603 m2g–1 (total pore volume is 1.03 cm3g–1) which is the highest measured surface area among all hydrazone COFs reported to date.The pore size distribution (PSD) was evaluated with non-local density functional theory (NLDFT). The experimental PSD exhibits a maximum at 3.8 nm (Fig. 2(c)), thereby verifying the theoretical pore diameter of 3.8 nm. Transmission electron microscopy images confirm the data derived from PXRD and sorption measurements. The hexagonal pore arrangement with pore distances of ≈3.4 nm is clearly visible, as well as the layered nanomorphology (Fig. 2(e)).

The diffuse reflectance UV/Vis spectrum of the yellow powder exhibits an absorption edge around 400 nm, with the absorption tail extending well beyond 600 nm (Fig. 3(a)). We estimate an optical band gap of roughly EB = 2.8 eV from the absorption edge, based on the Kubelka–Munk function. The TFPT–COF shows a pronounced red-shift of the absorption edge by 33 nm in comparison with the individual building blocks. In principle, the observed HOMO-LUMO gap of the TFPT–COF is large enough to enable water splitting through band gap excitation and at the same time small enough to harvest a significant portion of the visible light spectrum.

To investigate this possibility, we studied the light-induced hydrogen evolution mediated by Pt-modified TFPT–COF in the presence of a sacrificial electron donor as the photocatalytic system under visible light irradiation.

Photocatalytic hydrogen evolution

Hydrogen evolution was studied under standardized conditions and measured in the presence of the proton reduction catalyst (PRC) Pt, using a 10 vol% aqueous triethanolamine (TeoA) solution as sacrificial donor. A high hydrogen evolution rate was detected, with the amount of hydrogen evolved in the first five hours being as high as 1970 µmol h–1 g–1, corresponding to a quantum efficiency of 2.2%, while maximum QEs of up to 3.9% were obtained for individual batches (Fig. 3(a) and (b)). However, this high rate comes along with a quicker deactivation of the photocatalytic system. By reducing the amount of triethanolamine (1 vol%) and adjusting the suspension to pH = 7, stable hydrogen evolution for a longer time range (24 hours) was detected.

In conclusion, we have developed a new crystalline hydrazone-based TFPT-COF, which is the first COF to show photocatalytic hydrogen evolution under visible light irradiation in the presence of Pt as PRC. This framework is competitive with the best non-metal photocatalysts for hydrogen production and represents a lightweight, well-ordered model system, which in principle can be readily tuned – by replacement, expansion or chemical modification of its building blocks – to further study and optimize the underlying mechanism of hydrogen evolution mediated by the framework and to enhance its light harvesting capability. The triazine moieties in the TFPT–OF, which are likewise present in the recently developed triazine-based carbon nitride photocatalytic system PTI/Pt may point to an active role of the triazine unit in the photocatalytic process.

The development of COFs as tunable scaffolds for photocatalytic hydrogen evolution enables a general bottom-up approach toward designing tailor-made photosensitizers and photocatalysts with tunable optical and electronic properties, a goal we are currently pursuing in our lab. We expect this new application of COFs in photocatalysis to open new avenues to custom-made heterogeneous photocatalysts, and to direct and diversify the ongoing development of COFs for optoelectronic applications.